Field of the Invention
The present invention relates to ester-containing compositions stabilized against degradation and more particularly relates to aqueous dispersions of organic ester-containing compositions stabilized against degradation.
Brief Description of the Prior Art
Polyesters, particularly poly(ester-urethanes) are of particular interest in the coatings industry because of their excellent film properties. The poly(ester-urethanes), for example, can be made elastomeric in character and result in tough films of high tensile strength with high elongation. Unfortunately, the polyesters including the poly(ester-urethanes) are subject to degradation probably due to hydrolysis because of the ester groups contained in the polymer. This degradation results in a gradual deterioration of the excellent film properties. Degradation is particularly a problem when the polyester and poly(ester-urethane) are dispersed in an aqueous medium and the dispersions are stored at high temperature, that is, about 50.degree. C. and above.
It is known in the art that carbodiimides stabilize water-free polyesters and poly(ester-urethanes) against hydrolysis. See, for example, U.S. Pat. Nos. 3,193,522 and 3,193,523 to Neumann et al and U.S. Pat. No. 3,193,524 to Holschmidt et al, which relate to stabilization of polyesters, and U.S. Pat. No. 3,716,502 to Loew, which relates to the stabilization of poly(ester-urethanes). However, the prior art as exemplified by these references, for the most part, deals with polyester and poly(ester-urethane) molding or casting compositions which are prepared neat. Although the references also disclose that the carbodiimides can be used with polyester and poly(ester-urethane) coating compositions, these compositions are based on organic solvents and contain no added water.
It is not surprising that carbodiimides have not been used, prior to the present invention, to stabilize organic ester-containing compositions dispersed in aqueous medium. Carbodiimides are generally recognized in the art as being reactive with water. In fact, a well-known commercial supplier of carbodiimides cautions against contacting carbodiimides such as 2,2',6,6'-tetraisopropyl-diphenyl carbodiimide with water since they will react with water.
Surprisingly, it has been found that in spite of the reactivity of carbodiimides with water, they can be used to stabilize aqueous dispersions of organic ester-containing compositions, such as polyester and poly(ester-urethanes) against degradation.